Resistance welding electrodes are widely used in industry for spot welding steel, aluminum, copper, and various other metals. Some particular applications are the assembly of automobile bodies, truck bodies, and railroad car bodies. In each of these uses, a welding gun fitted with a pair of electrodes is moved in stages along a continuous weld path. At each stage, the electrodes are contacted with opposite sides of the workpieces to be welded, and an electric current is passed through the electrodes and workpieces. Electrical resistance of the metals produces localized heating which causes the workpieces to fuse at a weld site. The electrodes are then removed, and the workpieces are allowed to cool. To prevent the electrodes from softening and melting, cooling water is continuously circulated through them. When performed on a production line, these steps are each performed in rapid sequence and they are repeated at several different weld locations.
Conventional resistance welding electrodes are generally made from copper or copper alloys for low electrical resistivity and high current flow. The copper electrodes generally perform satisfactorily for a time, but they deteriorate rapidly when metal from the workpieces sticks to them. This phenomenon is called electrode "pick-up". Distortion in shape of the electrodes (called "mushrooming") may also pose a problem.
Some coatings aimed at overcoming the above-described problems have been developed in the prior art. For example, Huys U.S. Pat. No. 4,861,961 claims a welding electrode coated with titanium carbide. A tungsten carbide coating is also mentioned.
Lambert U.S. Pat. No. 2,431,334 discloses a welding electrode comprising a tungsten rod coated with tungsten carbide.
Engel U.S. Pat. No. 3,665,145 and Glagola U.S. Pat. No. 4,044,220 both disclose welding electrodes coated with nickel, beryllium, cobalt, iron, and alloys of such metals. Engel and Glagola did not suggest coating their welding electrodes with non-metals
It is a principal objective of the present invention to provide a coating for a resistance welding electrode tip comprising a layer of an electrically conductive ceramic material.
A related objective of the present invention is to coat a resistance welding electrode tip with a layer that preferably comprises tungsten disulfide.
A further objective of the invention is to provide a welding process utilizing a resistance welding electrode having a tip coated with an electrically conductive ceramic material.
Additional objectives and advantages of the present invention will become apparent to persons skilled in the art from the following detailed description.